Geologists show unprecedented warming in Lake Tanganyika

May 16, 2010

Lake Tanganyika is located in southeast Africa and is bordered by four of the poorest countries in the world. Credit: Brown University

Lake Tanganyika, the second oldest and the second-deepest lake in the world, could be in for some rough waters.

Geologists led by Brown University have determined the east African rift lake has experienced unprecedented warming during the last century, and its surface waters are the warmest on record. That finding is important, the scientists write in the journal Nature Geoscience, because the warm surface waters likely will affect fish stocks upon which millions of people in the region depend.

The team took core samples from the lakebed that laid out a 1,500-year history of the lake's surface temperature. The data showed the lake's surface temperature, 26 degrees Celsius (78.8°F), last measured in 2003, is the warmest the lake has been for a millennium and a half. The team also documented that Lake Tanganyika experienced its biggest temperature change in the 20th century, which has affected its unique ecosystem that relies upon the natural conveyance of nutrients from the depths to jumpstart the food chain upon which the fish survive.

"Our data show a consistent relationship between lake surface temperature and productivity (such as fish stocks)," said Jessica Tierney, a Brown graduate student who this spring earned her Ph.D. and is the paper's lead author. "As the lake gets warmer, we expect productivity to decline, and we expect that it will affect the [fishing] industry."

The research grew out of two coring expeditions sponsored by the Nyanza Project in 2001 and 2004. Cores were taken by Andrew Cohen, professor of geological sciences at the University of Arizona and director of the Nyanza project, and James Russell, professor of geological sciences at Brown, who is also Tierney's adviser.

Lake Tanganyika is bordered by Burundi, the Democratic Republic of Congo, Tanzania, and Zambia — four of the poorest countries in the world, according to the United Nations Human Development Index. An estimated 10 million people live near the lake, and they depend upon it for drinking water and for food. Fishing is a crucial component for the region's diet and livelihood: Up to 200,000 tons of sardines and four other fish species are harvested annually from Lake Tanganyika, a haul that makes up a significant portion of local residents' diets, according to a 2001 report by the Lake Tanganyika Biodiversity Project.

Researchers drilled cores into Lake Tanganyika to document the lake’s surface temperature for the last 1,500 years. They found unprecedented warming in the 20th century. Brown geologist James Russell, kneeling at drill head, led this core sampling mission in 2004. Credit: Kate Whittaker

Lake Tanganyika, one of the richest freshwater ecosystems in the world, is divided into two general levels. Most of the animal species live in the upper 100 meters, including the valuable sardines. Below that, the lake holds less and less oxygen, and at certain depths, it is anoxic, meaning it has no oxygen at all. What this all means is the lake is highly stratified and depends on wind to churn the waters and send nutrients from the depths toward the surface as food for algae, which supports the entire food web of the lake. But as Lake Tanganyika warms, the mixing of waters is lessened, the scientists find, meaning less nutrients are funneled from the depths toward the surface. Worse, more warming at the surface magnifies the difference in density between the two levels; even more wind is needed to churn the waters enough to ferry the nutrients toward the fish-dwelling upper layer.

The researchers' data show that during the last 1,500 years, intervals of prolonged warming and cooling are linked with low and high algal productivity, respectively, indicating a clear link between past temperature changes and biological productivity in the lake.

"The people throughout southcentral Africa depend on the fish from Lake Tanganyika as a crucial source of protein," Cohen noted. "This resource is likely threatened by the lake's unprecedented warming since the late 19th century and the associated loss of lake productivity."

Climate change models show a general warming in the region, which, if accurate, would cause even greater warming of the Lake Tanganyika's surface waters and more stratification in the lake as a whole. "So, as you move forward, you can imagine that density gradient increasing," Russell said.

Some researchers have posited that the declining fish stocks in Lake Tanganyika can be attributed mainly to overfishing, and Tierney and Russell say that may be a reason. But they note that the warming in the lake, and the lessened mixing of critical nutrients is exacerbating the stocks' decline, if not causing it in the first place. "It's almost impossible for it not to," Russell said.

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38 comments

Fifteen years ago, I told my brother there are basically two sources of heat for our planet--above (the sun) and below (radioactive decay within the core. If the climate is warming, then one of these two must be responsible. If Lake Tangyanika is warming up, it is not because the globe is warming, but the other way around. Water is hugely more dense than air. Water can warm (or cool) air; but air will have little effect on water. Obviously (note all the volcanic activity of the past hundred years, including volcanoes blowing up) the earth's center is warming up. And we, like idiots, think that "cap and trade" restrictions on CO2 will make a difference. Only the devil can decieve like this.

Is this warming due to habitat change (forest removal) surrounding the lake, since we know that replacing forest with fields causes significant warming and drying? The cause? Too many people for the habitat? No it couldn't be that, must be CO2.

"The data showed the lake's surface temperature, 26 degrees Celsius (78.8°F), last measured in 2003, is the warmest the lake has been for a millennium and a half."

When statements like this are made without indicating either quantity or quality of the data, I always wonder how far the study can be trusted. 26 degrees is the warmest in 1500 years, how much warmer? And what is the standard error? You know? Like "the average surface water temperature over the last 1.5ky is 23.2+-1.5C", or 23.2, -1, +2C.

I question whether the average surface water temperature can even be quantified to a tenth of a degree. How about providing some of the statistical information those fancy peer reviewed papers seem to always have?

Please explain how the Earth's center is suddenly-after ~ 4.5 billion years, warming up. I'm sure there are many, many people out there who would be VERY interested to know.

Think about a lava lamp. Convection. Circulation. Also the lake is part of the African rift system. You know, where the continent is splitting apart? If you google earth it you will see volcanos, old and new, all around the area.

"Climate change models show a general warming in the region, which, if accurate, would cause even greater warming of the Lake Tanganyika's surface waters and more stratification in the lake as a whole."

The article does not specifically blame climate change for the lake temperature but since it does mention it and implies that the warming is surface warming which would decrease the mixing/biotic productivity, it is written as a climate issue - whether this climate change is caused by solar variations or magic gases and human malevolence. What is the term for someone who calls people "trolls" without cause?

Given that the lake is formed by the continent ripping apart, it might be nice if those responsible for raising this issue explain how they ruled out volcanic energy from the change in temperature.

Lino235 - not only are you ill informed (as Caliban eluded to) You are also an internet troll.

Nobody likes trolls (except maybe themselves)

I cannot speak for Lino but I believe it is pointing out some of the obvious deficiencies in the AGW propaganda.

For instance, how can "cap and trade" make any difference in a geologically active world? Answer, it cannot. Why? Because mankind's contribution (~1-3%) and hence, the actual amount of carbon "moved around" by such schemes falls somewhere deep within the geologic system's background noise. IOW, differences in geologic activity swamp any human carbon signal.

At least I think that is where he was going, well maybe not, but it is the point I wanted to make.

He isn't wrong about the two sources of heat either. The estimated blackbody temp of Earth is -18C, with just the sun.

...there are basically two sources of heat for our planet--above (the sun) and below (radioactive decay within the core. If the climate is warming, then one of these two must be responsible. . . . And ... like idiots, think that "cap and trade" restrictions on CO2 will make a difference. Only the devil can decieve like this.

I agree with your "like idiots" part.#1 Just turn up you thermostat and you'll see other sources of heat on earth. We can discuss tides etc later.

#2 I ran on a really cold day. My body was producing more heat than at home, but I was warmer at home. How can this be?? Oh, my home is insulated. CO2 is a kind of insulator. Think about it.

#3 So exactly how did the interior of the earth suddenly get hotter than before?? Actually, it is very slowly getting cooler. The crust is nearly all solid by now. The rate of volcanism has not changed.

Think about a lava lamp. Convection. Circulation. Also the lake is part of the African rift system. You know, where the continent is splitting apart? If you google earth it you will see volcanos, old and new, all around the area.

Think about this convection mechanism having been in effect since very early in earth's history, and being the driving force behind plate tectonics, your rift valley being only one of several manifestions of this, and it, itself a few millions of years in the making.

The Earth would have started at maximum thermal density, x, due to compression, exothermic chemical reactions, radioactive decay. In say 4 billion years, there have most definitely been losses in several of those categories, and now thermal density is less, correct? How then is Earth's interior heating up?

If it's a case of magma plume heating, that would leave some evidence in terms of heating of the lowest water layer, and lake sediments/floor- none of which is reported here.

Given That the entire great lakes region of Africa is geologicaly active as has been said. It is very likely that the cause of the increased temperature, if indeed there is an increase in temperature. Is the result of geological activity. Climate change models are just that, models. Usually based on someones interpretation of data in a way which will ultimately confirm the result that they want.

As far as the AGW so called'debate' which it is not. Over the last 650thousand years there have been 7 major glaciations and 7 warm periods. The preiods of glaciation total approx 400 thousand years the warm periods approx 250 thousand years. all of the glaciation periods were followed by warm periods including the present. These periods when the earth was warmed could reasonable be called periods of 'Global Warming' mainly because that is exactly what they were/are. The only problem is that during 6 of them there were no humans around to be blamed for it.

Shootist

When statements like this are made without indicating either quantity or quality of the data, I always wonder how far the study can be trusted. 26 degrees is the warmest in 1500 years, how much warmer? And what is the standard error? You know? Like "the average surface water temperature over the last 1.5ky is 23.2+-1.5C", or 23.2, -1, +2C...How about providing some of the statistical information those fancy peer reviewed papers seem to always have?

How about reading the actual article if you want the details, instead of a news site? "Late-twentieth-century warming in Lake Tanganyika unprecedented since AD 500" 16 May 2010 | doi:10.1038/ngeo865

Yes, volcanic activity has long been ruled out. If the lake was warming from volcanic activity (from contact with magma below the lake or some such), the deepest parts of the lake would be warming more rapidly. There would also be some real significant and obvious indicators in the water's chemistry, etc. etc.

This paper doesn't deal with air temperatures causing surface warming as much as other articles on Lake Tanganyika, but it does show that the lake is warming from the surface. It also explains how this can happen with such a deep lake (severe stratification). It's actually a good article.

The main two points this paper demonstrates are that 1) the lake's productivity is going to be negatively affected by increased lake temperatures and 2) these temperatures are significantly outside variance

Thank you for that last comment. There are a bunch of trolls posting here, claiming absurd crap and shouting about bias, when the article itself clearly accounts for their claims (having taken no bias about the cause of the warming) and proven them wrong.

The researchers' data show that during the last 1,500 years, intervals of prolonged warming and cooling are linked with low and high algal productivity, respectively, indicating a clear link between past temperature changes and biological productivity in the lake.

Can anyone perhaps spell out how they measured the temperatures for a period of 1500y? Not so clear as to where that comes from. Thanks.

This sounds like horrible science. They are jumping to a number of conclusions and making a lot of arbitrary statements in this article. Who lets this stuff get into publication anyway? It's no wonder the public is so uninformed when this kind of junk is released. I'll bet the mainstream media will run with this one. It fits thier agenda perfectly and it's hard to argue against a story that doesn't have much real science to argue against. I especially like the part where it says that the whole thing is based on "climate models" and a single surface temperature measurement that is several years old. WONDERFUL. This article should be taken off this site. It's pure junk.

Please explain how the Earth's center is suddenly-after ~ 4.5 billion years, warming up. I'm sure there are many, many people out there who would be VERY interested to know.

Actually I can do this, but it requires AGW to be accurate, or at least GW to be accurate.There's a hypothesis kicking around that increases in oceanic volume put additional pressure on the plates of the planet leading to greater incidences of volcanism and tectonic shift events.

So with AGCC/AGW/GW we're seeing ice flows increase the pressure of water upon the crust, creating greater incidents of volcanism, blasting cooling elements into the atmosphere.

If we are accelerating warming, which I'm not saying we certainly are (although I get closer to doing so each day), then we're also accelerating volcanic and tectonic events. If this is a natural feedback then we'd be increasing the oscillations of volcanism and tectonic activity, which will be to our detriment.

P.S. They don't even mention tidal action, which is a major factor in the mixing of stratified water bodies of this kind. Ground water runoff and tributary inflow are another source of mixing because water flowing in from streams and rivers is dense with particles and solutes. I would also like to know if the lake is controlled by a hydroelectric dam. That would be a major factor as well. Would anyone care to look that up?

Changes in air temperature MAY change the rate of mixing, but I fail to see any indication that the researchers here have found reasonable evidence of the magnitude of effect. Doesn't that matter to anyone here?

Stats abound in the full text. I imagine it's entirely over the head of the average reader of sites like PhysOrg, though, and you should know better than to demand full stats from news sites.

Thanks for the reply. Having access to "full papers" would be nice but it is outside my budget right now.

And I didn't ask for "full stats", just some idea of how the numbers compare. The news article contained no real information. And no, I do not believe stating averages with error bars is beyond the average reader (though some may attribute surface warming to "The Electric Universe" or some such).

yes, lenquold100. That is correct. The comments about geothermal warming are baseless and only distract from the relevant facts. As with a mid-oceanic seafloor vent, the temperature of water surrounding a vent will be quite hot, but the water only a short distance away is still cold. Ignore those people.

The portion of the lake at the bottom hasn't mixed with the surface of the lake in thousands of years and is called "fosile water". The mixing occurs very near the surface only, and it is surface conditions only which apply here. The factors at work include wind, rain, sun, tides, inflow, crop fertilizer concentrations, sewage and industrial pollutants, trash dumping, natural marine biology and probably sereral other factors. Before the researchers here can attribute a decrease in marine biological productivity to global warming, they must first quantify and eliminate the potential statistical noise from all those other factors. How did they do that? Did they do that?

They are jumping to a number of conclusions and making a lot of arbitrary statements in this article...I especially like the part where it says that the whole thing is based on "climate models" and a single surface temperature measurement that is several years old.

*sigh* this comment is pure junk. This article is typical science reporting. The actual journal article, published Nature Geoscience, is certainly not junk. Your comments about this news article aren't even correct, let alone your extension of those to the paper itself.

The "whole thing" isn't based on climate models--a small portion is. This article/paper is about how increased temps increase stratification thereby reducing productivity. They have quite a lot of evidence and data for this conclusion.

It's a blatant lie to claim this paper is based on a single surface temperature measurement. LST numbers were done with a leave-one-out calibration estimate.

Thanks for the reply. Having access to "full papers" would be nice but it is outside my budget right now.

Understandable, I'm not a big fan of the pay access models myself. I'm lucky to have good database access, but when I didn't I had friends who work in research send me the articles I wanted to read. I know, I know, copyright violation.

Fair enough on your request for more detailed stats. I wish science journalists would report a lot more than they do, but it's pretty obvious from comments around here that people don't understand the little information they already put out. More info would be nice, but it's the exception to the rule in journalism. It's not fair to fault the actual scientists for weak reporting, as many are doing here.

...The factors at work include wind, rain, sun, tides, inflow, crop fertilizer concentrations, sewage and industrial pollutants, trash dumping, natural marine biology and probably sereral other factors...they must first quantify and eliminate the potential statistical noise from all those other factors. How did they do that? Did they do that?

Thanks for having a well thought-out question; unfortunately, I can't answer it in 1,000 characters or less. They did address myriad other factors, most of which can be found in pg. 3 of the paper. Rainfall was the most likely alternative force, but its correlation was much weaker than LST-BSi.

The rainfall correllation is probably systemic due to the water vapor - temperature feedback loop, so a coincidence of temp and rainfall data isn't surprising. That feedback loop should also mean stronger denser wind as temperature increases which would have a positive mixing effect, I would expect. I wonder if the effect of temperature on mixing would be the same if the lake weren't surrounded by mountains?

How could temperature increase while amount of fuel went down? Well, it isn't so simple, because as nuclear material decays it produces daughter products, and some of those products decay too, and they have different energy values for the amount of energy released when they decay.

Anyway, the total amount of energy humans use each year is less than one ten thousandth of what the earth recieves from the Sun in a year.

It would also take hundreds of years worth of 2010 human energy production to equal the temperature change recorded for this 16 year span.

How could temperature increase while amount of fuel went down? Well, it isn't so simple, because as nuclear material decays it produces daughter products, and some of those products decay too, and they have different energy values for the amount of energy released when they decay.

Really? Show us the numbers.

Anyway, the total amount of energy humans use each year is less than one ten thousandth of what the earth recieves from the Sun in a year.

It would also take hundreds of years worth of 2010 human energy production to equal the temperature change recorded for this 16 year span.

Nice try. It's not human energy consumption, but PRODUCTION of greenhouse gases, and the resultive radiative forcing that is of concern here. You have failed to cloud the issue.

If heated from below, the warmest aprt of the lake would be the lowest. It isn't. Stop trolling, all you non-scientific agenda bunch.

The only time the water at a lake's bottom is warmer than its surface is in the winter when surface temperatures are below 4C.Warm water rises, cold water sinks.

I know that warm water rises (which would be decreasing stratification), I was oversimplifying. The warmest water would indeed be wherever water is being directly heated by magma or some other "volcanic heat". That water would then rise, mixing with cooler waters, and be heating the lake from beneath. You wouldn't have an extremely stratified lake with ever-increasing surface temperatures.

You're really stretching if you're still trying to back the volcanic activity claim.

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